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The domestic dog has become the fourth mammal species - after humans, rats and mice - to have its genome mapped.

A U.S. team led by Dr Ewen Kirkness, of the Institute for Genomic Research in Rockville, Maryland, report in today's issue of the journal Science, that they have produced a "rough sketch" of the dog genome, based on an analysis of genes from a standard poodle.

The researchers acknowledge the draft sequence does not yet cover all dog genes but argue their relatively cheap approach will yield significant information about the genome's content and its relationship with the human and rodent genomes.

"The progress in dog genetics has been astounding." say Stephen O'Brien and William Murphy, of the Laboratory of Genomic Diversity at the U.S. National Cancer Institute, in a related commentary in the journal. "With the dog sequence, genomics ventures from the laboratory haven of traditional rodent models of disease, into the living room with an annotated whole-genome sequence of man's best friend."

The team found that dogs and humans are more like each other genetically than either is to a mouse. The study suggests that dogs have at least 18,500 genes in common with a total of some 25,000 human genes so far identified.

The researchers chose not to use the more expensive method of generating a "high-level" genome sequence that uses a relatively large number of overlapping pieces of DNA sequence, so that the genome would be actually covered as many as eight times.

Instead, they covered the poodle's genome just 1.5 times, meaning that slightly less than 80% of all the genes were identified. Even so, it appears that the dog genome is significantly smaller than the human one.

O'Brien and Murphy praise the choice of the dog as an early priority in sequencing the genomes of other mammals because it will enable useful comparisons to be made and because so much is already known scientifically about dogs and the diseases they have in common with humans.

They point out that dogs have been domesticated since the dawn of agriculture, 10,000 to 15,000 years ago, and today exist in more than 400 distinct breeds with a great variety of physical and behavioural genetic differences suitable for genetic exploration.

"Dogs enjoy a medical surveillance and clinical literature second only to humans, succumbing to 360 genetic diseases that have human counterparts," comment O'Brien and Murphy. "Dogs have been beneficial for standard pharmaceutical safety assessment and also for ground-breaking gene therapy successes."

They also note that rapid progress is being made in genomic sequencing, a trend that promises to add significantly to basic knowledge about human genetics and to open up new treatment possibilities for genetically linked diseases.

"For mammals, the genome sequence derby is heating up," they say. "Human, mouse, rat, and now dog have produced appreciable genome sequence, with chimp, cow, and rhesus macaque scheduled for full-genome sequence by 2005."

According to one recent estimate, sequencing centres in the U.S. alone could produce the equivalent of 192 dog-sized genomes.

O'Brien and Murphy note that a recent meeting of researchers who specialise in the field recommended 18 primate and 28 non-primate species to be considered next for whole-genome sequence analysis.

"Although costly, there is little doubt that comparative genomics of the mammalian radiations will greatly inform human biology as well as that of the 4,600 species of mammals with which we share the planet," they say.